In a turbine engine, fan blade platforms need to perform several functions. From an aerodynamic point of view, such platforms serve primarily to define the air flow passage. They must also be capable of withstanding large forces without deforming and while remaining secured to the disk that carries them.
In order to satisfy these various requirements, certain configurations have been proposed in which platforms possess a first portion serving to define the air flow passage and to retain the platform while the engine is rotating, and a second portion serving to limit deformation of the first portion under the effects of centrifugal forces and to hold the platform in position when the engine is stationary.
In existing solutions, the platform may be in the form of a box having a two-dimensional passage wall that is retained downstream by a drum and upstream by a shroud, with upstream retention by the shroud being provided over the tooth of the fan disk (a flange of the shroud serves to block the upstream end of the platform both axially and radially).
Such upstream retention provided over the tooth of the disk by means of a shroud presents the drawback of imposing a large hub ratio, where the hub ratio is the ratio of the radius measured between the axis of rotation and the point of the leading edge of the blade that is flush with the surface of the platform, over the radius measured between the axis of rotation and the outermost point of the leading edge.
In order to optimize the performance of the fan, and more generally of the engine, it is desirable to have an assembly obtained by assembling a separate fan blade platform on a fan disk that presents a hub ratio that is as small as possible.